1. Field of the Invention
This invention relates to a method of enhancing mutant enzyme activities in lysosomal storage disorders by administration of competitive inhibitors of the enzymes, in particular, imino sugars and related compounds.
2. Background Information
Proteins are synthesized in the cytoplasm, and the newly synthesized proteins are secreted into the lumen of the endoplasmic reticulum (ER) in a largely unfolded state. In general, protein folding is an event governed by a principle of self assembly. The tendency of proteins to fold into their native (active) conformation is contained in their amino acid sequences (1). In vitro, the primary structure folds into secondary structres of α-helices and β-sheets coupled with hydrophobic collapse in the formation of biologically active tertiarystructure which also gains increased conformational stability. However, the folding of protein in vivo is rather complicated, because the combination of ambient temperature and high protein concentration stimulates the process of aggregation, in which amino acids normally buried in the hydrophobic core interact with their neighbors non-specifically. To avoid this problem, protein folding is usually facilitated by a special group of proteins called molecular chaperones which prevent nascent polypeptide chains from aggregating, and bind to protein so that the protein refolds in the active state (2).
Molecular chaperones are present in virtually all types of cells and in most cellular compartments. Some are involved in the transport of proteins and in supporting cells to survive under stresses such as heat shock and glucose starvation. Among the molecular chaperones (3–6), BIP (immunoglobulin heavy-chain binding protein, Grp78) is the best characterized chaperone of the ER (7). Like other molecular chaperones, BIP interacts with many secretory and membrane proteins within the ER throughout their maturation, although the interaction is normally weak and short-lived when the folding proceeds smoothly. Once the native protein conformation is achieved, the molecular chaperone no longer binds. However, the interaction between BIP and those proteins that fail to fold, assemble or be properly glycosylated becomes stable, and usually leads to degradation of these proteins through the ubiquitin pathway. This process serves as a “quality control” system in the ER which ensures that only properly folded and assembled proteins are transported to the Golgi complex for further maturation, and those improperly folded proteins are retained for subsequent degradation (8).
In many hereditary disorders, mutant gene products are structurally altered and may not fold correctly, signalling the quality control system to retain and degrade them in situ. This process may contribute significantly to the protein deficiency, although the function of the protein may have been only partially impaired (9–12). For example, the most common mutation in cystic fibrosis, a deletion of phenylalanine-508 (ΔF508) in the CFTR protein which functions as a chloride channel in the plasma membrane, results in misfolding and retardation of the ΔF508-CFTR protein in the ER, and subsequent degradation by the cytosolic proteasome system (13–14), even though it retains almost full biologic activity when inserted into plasma membranes (15). The list of diseases caused by mutations that alter protein folding is increasing, and it includes α1-antitrypsin deficiency (16–17), familial hypercholesterolemia (18), Alzheimer's disease (18a), Marfan syndrome (19), osteogenesis imperfecta (20), carbohydrate-deficient glycoprotein syndrome (21), and Maroteaux-Lamy syndrome (22).
Lysosomal storage disorders are a group of diseases resulting from the abnormal metabolism of various substrates, including glycosphingolipids, glycogen, mucopolysaccharides and glycoproteins. The metabolism of exo- and endogenous high molecular weight compounds normally occurs in the lysosomes, and the process is normally regulated in a stepwise process by degradation enzymes. Therefore, a deficient activity in one enzyme may impair the process, resulting in an accumulation of particular substrates. Most of these diseases can be clinically classified into subtypes: i) infantile-onset; ii) juvenile-onset; or iii) late-onset. The infantile-onset forms are often the most severe usually with no residual enzyme activity. The later-onset forms are often milder with low, but often detectable residual enzyme activity. The severity of the juvenile-onset forms are in between the infantile-onset and late-onset forms. Table 1 contains a list of a number of known lysosomal storage disorders and their associated defective enzymes. In the adult-onset forms of lysosomal storage disorders listed in Table 1, certain mutations cause instability of the encoded protein.
TABLE 1Lysosomal storage disorders.Lysosomal storage disorderDefective enzymePompe diseaseAcid α-glucosidaseGaucher diseaseAcid β-glucosidse, or glucocerebrosidaseFabry diseaseα-Galactosidase AGM1-gangliosidosisAcid β-galactosidaseTay-Sachs diseaseβ-Hexosaminidase ASandhoff diseaseβ-Hexosaminidase BNiemann-Pick diseaseAcid sphingomyelinaseKrabbe diseaseGalactocerebrosidaseFarber diseaseAcid ceramidaseMetachromaticArylsulfatase AleukodystrophyHurler-Scheie diseaseα-L-IduronidaseHunter diseaseIduronate-2-sulfataseSanfilippo disease AHeparan N-sulfataseSanfilippo disease Bα-N-AcetylglucosaminidaseSanfilippo disease CAcetyl-CoA: α-glucosaminide N-acetyltransferaseSanfilippo disease DN-Acetylglucosamine-6-sulfate sulfataseMorquio disease AN-Acetylgalactosamine-6-sulfate sulfataseMorquio disease BAcid β-galactosidaseMaroteaux-Lamy diseaseArylsulfatase BSly diseaseβ-Glucuronidaseα-MannosidosisAcid α-mannosidaseβ-MannosidosisAcid β-mannosidaseFucosidosisAcid α-L-fucosidaseSialidosisSialidaseSchindler-Kanzaki diseaseα-N-acetylgalactosaminidase
In their earlier filed patent application (U.S. application Ser. No. 09/087,804), the present inventors proposed a novel therapeutic strategy for Fabry disease, a lysosomal storage disorder caused by deficient lysosomal α-galactosidase A (α-Gal A) activity in which certain mutations encoded mutant proteins which have folding defects. The application presented evidence demonstrating that 1-deoxygalactonojirimycin (DGJ), a potent competitive inhibitor of α-Gal A, effectively increased in vitro stability of a mutant α-Gal A (R301Q) at neutral pH and enhanced the mutant enzyme activity in lymphoblasts established from Fabry patients with the R301Q or Q279E mutations. Furthermore, oral administration of DGJ to trangenic mice overexpressing a mutant (R301Q) α-Gal A substantially elevated the enzyme activity in major organs (24).
The principle of this strategy is as follows. Since the mutant enzyme protein appears to fold improperly in the ER where pH is neutral, as evidenced by its instability at pH 7 in vitro (25), the enzyme protein would be retarded in the normal transport pathway (ER→Golgi apparatus→endosome→lysosome) and subjected to rapid degradation. In contrast, an enzyme protein with a proper folding conformation could be efficiently transported to the lysosomes and remain active, because the enzyme is more stable below pH 5. Therefore, a functional compound which is able to induce a stable molecular conformation of the enzyme is expected to serve as a “chemical chaperone” for the mutant protein to stabilize the mutant protein in a proper conformation for transport to the lysosomes. Some inhibitors of an enzyme are known to occupy the catalytic center of enzyme, resulting in stabilization of its conformation in vitro, they may also serve as “chemical chaperones” to enforce the proper folding of enzyme in vivo, thus rescue the mutant enzyme from the ER quality control system. It is noted that while this is believed to be the mechanism of operation of the present invention, the success of the invention is not dependent upon this being the correct mechanism.